Radiation describes any process in which energy emitted by one body travels through a medium or space to ultimately be absorbed by another body. Where nuclear materials are concerned, radiation may be further considered to be the energy being transmitted as particles or waves, particularly the gamma rays emitted during nuclear decay. Some types of radiation possess enough energy to ionize particles, which may involve an electron being knocked out of an atom's electron shell, thereby giving it a positive charge. These radiation effects are often disruptive in biological systems and can result in mutations and cancer.
The scientific unit of measurement for a dose of radiation depends not only upon the system of measurement—English or Metric—but also upon the aspect of radiation that is being considered. For the amount of radiation being emitted by a radioactive material into the environment, the conventional unit is the “Curie” (Ci), which is named after Marie Curie for her research on the highly radioactive element radium, and using the System Internationale (SI, or metric system), the units are the Becquerel (Bq). For example, the amount of radioactive material estimated to have been accidentally released by the Chernobyl nuclear power plant in the Soviet Union in 1986 was 81 million CI of radioactive cesium.
Where a person is exposed to radiation and energy is deposited into the tissues of the body, the amount of energy deposited per unit of weight of human tissue is known as the absorbed dose, which is measured either in the conventional units—the “Rad” (radiation absorbed dose), or in the SI units—the “Gray” (Gy). However, since different tissues and organs of the body have varying sensitivities to radiation exposure, the actual risk from radiation exposure to different parts of the body varies, and is reflected in two other types of measurements.
The measure of a dose of radiation in terms of its potential to cause damage is known as the Equivalent Dose (HT), and is defined as:HT=WRDT,R Where DT, R is the absorbed dose delivered by radiation type R averaged over a tissue or organ T; and WR is the radiation weighting factor for the radiation type R, which is a measure of the biological damage producing potential of the radiation R. The units for the dose equivalent is the “rem” (the roentgen equivalent in man), and in the SI system, it is the “Sievert,” which is more conveniently expressed as the millisievert (mSv, or 10−3 Sievert).
The Effective Dose (E) is defined as the summation of the tissue equivalent doses each being multiplied by the appropriate tissue weighting factor, Wt, to indicate the combination of different doses to several different tissues, as shown by the following formula:E=ΣWTHT (see “Relative Biological Effectiveness (RBE), Quality Factor (Q) and Radiation Weighting Factor (wR),” in the Oxford Journal, Radiation Protection Dosimetry, Edited by J. Valentin, Published by: ICRP Publication 92, Annals of the ICRP, Vol: 33(4), 117 pp (2003), available at http://rpd.oxfordjournals.org/content/108/3/270.extract, the disclosures of which are incorporated herein by reference).
Unbeknownst to many of us, the average person is exposed to radiation throughout the year, even while simply undergoing the activities of routine daily living, because of natural background radiation. Natural background radiation comes from two primary sources—terrestrial sources, and cosmic radiation. The world-wide average “background” dose of radiation received by a human being is 2.4 mSv per year, which is mostly from cosmic radiation originating principally from the sun, and due to natural radionuclide found in our environment, such as the radon gas released from the earth's crust, which may attach to airborne dust. The average person in the United States receives approximately 3 mSv per year, while individuals in certain areas around the world, such as the Flinders mountain range in southern Australia and the region around Ramsar, Iran, receive significantly higher annual doses.
The annual limit on intake (ALI) is the derived limit for the amount of radioactive material that is taken into a body of an adult by inhalation or ingestion in a single year. The ALI is the smaller value of either the intake of a given radionuclide in a year by a reference man that would result in a committed effective dose equivalent of 0.05 Sv (50 mSv or 5 rems), or a committed dose equivalent of 0.5 Sv (500 mSv or 50 rems) to any individual organ or tissue.
Radiation exposure is a major concern in both adults and children. Radiation effects on a person begins at the lowest level and progresses upward, in that radiation causes ionization of atoms, which may affect molecules in a cell, which may then effect the cell itself, which could affect tissues, which may affect an entire organ, which may thereafter affect the entire body. The health effects due to exposure to radiation may be categorized as direct effects or indirect effects. Where direct effects occur, radiation has interacted directly with an atom of the DNA molecule or other critical cell component. If a sufficient number of atoms are affected, the life-sustaining nature of the cell may be destroyed, such as where atoms in the chromosomes become damaged and no longer replicate properly, or where the information in the DNA molecule has been significantly altered. Indirect radiation effects occur because a cell is mostly water, which reduces the probability that the radiation will interact with the DNA molecule, but the radiation may break the bonds of a water molecule, producing hydrogen (H) and hydroxyl (OH) fragments, which may combine with other fragments or ions to form toxic compounds, such as hydrogen peroxide (H2O2), which can lead to the destruction of the cell.
In some cases, where a cell or many cells have been damaged, they may nonetheless be able to completely repair themselves. If the damage is severe enough, the cells may die, however, they may be able to partially repair themselves, but thereafter their children cells may be damaged and be unable to survive. It is also possible for one or more cells to be damaged to the point where mutation occurs, and reproduction perpetuates the mutation, which could lead to a tumor. The factors that determine these effects are the (1) dose rate, (2) total dose received, (3) energy of the radiation, (4) area of the body exposed, (5) individual sensitivity, and (6) cell sensitivity.
In addition to natural background radiation, where even a round-trip transcontinental flight may result in an additional 0.55 mSv of radiation exposure, an individual's annul dose of radiation may be received from many manmade sources. These manmade sources may include: the tritium in self-luminous watches and dials; the thorium using in the incandescent mantle of camping lanterns; the thorium found in tobacco; the Americium found in smoke detectors; the cesium used in coal plants to determine the ash and moisture content of its burnt fuel, as well as the uranium and thorium released in the fly ash of the burnt coal; the x-ray radiation produced by television cathode ray tubes; airport x-ray systems; and the radiation from normal operations of nuclear power plants and nuclear processing facilities, as well as accidents occurring therein. Moreover, manmade radiation exposure may occur to medical patients undergoing radiographic procedures—diagnostic procedures or treatments.
Treatment in the form of radiation therapy, also known as radiation oncology (XRT), may involve the targeting of ionizing radiation onto malignant cancer cells to be curative, adjuvant (preventative), or palliative (providing symptomatic relief through local disease control). The goal of these treatments being to damage the DNA of the cancer cells using either photon or charged particle energy. For curative treatments, a typical dose may be 20 to 40 GY for lymphomas, or 60 to 80 GY for epithelial tumors. Preventative treatments may typically total 45-60 Gy, in 1.8 to 2.0 Gy fractions.
Radiographic imaging may occur in many different forms. In nuclear medicine imaging, radiopharmaceuticals may be taken internally (intravenously or orally), which permits the emitted radiation to be imaged. Conversely, other techniques of diagnostic nuclear medicine involve passing external radiation through a person's body to form an image, with these methods usually being organ or tissue specific. One of the most common methods has been the “X-ray,” with its use beginning around 1895. X-rays are electromagnetic radiation having wavelengths in the range of 0.01 to 10 nanometers.
Diagnostic x-ray examinations may be separated into different categories—low dose examinations and high dose examinations. For low dose exams, such as a simple chest x-rays, the decision to have one is easy, since exposure is minimal. The radiation exposure from one chest x-ray is roughly 0.1 to 0.2 mSv, and can be compared to the amount of radiation exposure that an individual experiences from natural surroundings in ten days.
Anatomical imaging from higher dose x-ray examinations may occur through the use of a CT scan (Computed Tomography scan), which involves digital geometry processing to create a three-dimensional image of the inside of an object, from a large series of two-dimensional x-ray images taken around a single axis of rotation. The CT scan was introduced in the mid-1970s, and has since been widely adopted as a valuable medical tool for diagnosing disease, trauma, or abnormality, and for planning therapeutic regimens. However, it exposes the patient to significantly higher doses of radiation, with the radiation dose from one CT scan possibly being equivalent to the dose from hundreds of chest x-rays. For example, where a normal chest x-ray may result in 0.1 to 0.2 mSv of exposure, a head CT may result in 1.0 to 2.0 mSv; a chest CT may result in 5-7 mSv; an abdomen and pelvic CT may result in 8 to 14 mSv; a coronary artery CTA may result in 5 to 15 mSv; and a neonatal abdomen CT may result in 20 mSv of exposure. For a person having a medical need, the lifesaving benefits of administering a therapeutic or diagnostic CT procedure will likely far outweighs the cancer risk associated with the exam.
Although the amount of radiation used in diagnostic nuclear medicine is deliberately maintained within a safe limit, and adheres to the ALARA (“as low as reasonably possible”) principal proffered by the International Commission on Radiological Protection (ICRP), the risk of developing a cancer increases as a patient's frequency of administration increases. In addition to the profusion of x-rays being used to form the 3-dimensional CT image, the CT scan may involve the use of a contrast agent-material such as barium sulfate or iodine, which may further increase the risks of causing cancer.
Thus, there is a commitment in the field of radiology to avoid all unnecessary exposures, and when unavoidable, to make every reasonable effort to reduce exposure, which may include exposure to the isotopes of iodine (I-131), technetium (Tc-99m), cobalt (Co-60), iridium (Ir-192), cesium (Cs-137), and others. It is therefore a good idea for a patient that has had, or continues to have, frequent radiological exposure, to have access to a record of his or her history in order to help his or her healthcare professional make an informed decision about further examinations and treatment programs. Because a patient may receive exposures from a number of medical professionals, the invention herein discloses a unique method of accounting for all such exposures, and of protecting a patient from unnecessarily high cumulative doses of radiation.